CHAPTER 3: SUBGRADE, SUBBASES, and BASES

Table of Contents

CHAPTER 3: SUBGRADE, SUBBASES, and BASES

300 GENERAL

301 LIME TREATED SUBGRADE 301-2 MATERIAL 301-3 CONSTRUCTION REQUIREMENTS 301-3.01 PREPARATION OF ROADBED 301-3.02 APPLICATION OF LIME 301-3.03 MIXING 301-3.04 COMPACTION AND FINISHING 301-3.06 SAFETY PROGRAM

302 CEMENT TREATED SUBGRADE 302-3 CONSTRUCTION REQUIREMENTS 302-3.03 MIXING

303 AGGREGATE SUBBASE AND AGGREGATE BASE 303-2 MATERIALS 303-3 CONSTRUCTION REQUIREMENTS 303-3.01 PLACEMENT 303-3.02 COMPACTION 303-3.03 FINISHING 303-3.04 CONTRACTOR QUALITY CONTROL

304 CEMENT TREATED BASE 304-3 CONSTRUCTION REQUIREMENTS 304-3.02 MIXING 304-3.03 COMPACTING AND FINISHING 304-3.06 CURING SEAL

305 LEAN CONCRETE BASE 305-3 CONSTRUCTION REQUIREMENTS 305-3.08 PLACING AND FINISHING 305-3.09 CURING 305-3.12 OPENING TO TRAFFIC

306 GEOGRID BASE REINFORCEMENT 306-3 CONSTRUCTION REQUIREMENTS 306-3.04 GEOGRID PLACEMENT

307 GEOCOMPOSITE EDGE DRAIN 307-3 CONSTRUCTION REQUIREMENTS 307-3.04 CONSTRUCTION METHOD

REFERENCES AND ADDITIONAL INFORMATION

BLANK FORMS There are no blank forms associated with this chapter at the present time.

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300 GENERAL

A highway pavement is said to be only as good as the base under it. This material is generally of lower quality than any course placed over it, but it still carries and distributes the wheel loads. The need to be concerned with its quality and finish is very important.

The thickness of the future pavement is determined by the soil-support capabilities of the underlying subgrade. The soil-support value of this material can be found in the Final Materials Design Report.

Verification testing of the subgrade material by the field office should be done quickly so that possible design changes in overlying structure sections can be analyzed. Design adjustments at this time could be very expensive. Early verification of the subgrade will provide the needed time to consider all options so that quality versus economy can be evaluated.

When the Contractor is working a pit or quarry, quality control is the Contractor's responsibility. Nevertheless, The Resident Engineer (RE) should be aware of the methods used to work the pit and produce the materials.

Most material sources contain materials that should be avoided or wasted. Blending or selection of material from various areas of the source may be required.

The Resident Engineer cannot direct that a Contractor furnished source be worked in a particular manner, but can advise the Contractor when the methods or materials may not produce an acceptable product. Comments of this type should be supported by test results.

The subgrade will be finished using natural materials. However, it may be treated with lime or cement if conditions require. Cement is normally used for the binding agent when the subgrade materials are granular, or have a very low P.I. content. In cases of high P.I., or higher degree of clay particles, lime would normally be used as the binding agent.

Geo-composite materials are being used favorably by the department throughout the state to achieve acceptable subgrade stabilization. Approval should be obtained from ADOT Materials Group prior to using geo-composite materials. There are many different types of geo-composite materials. ADOT Materials Group will select the proper geo-composite material for your application.

The time needed to check the grade for acceptance usually depends on the Contractor's method of operation. Checking grade for compliance with finishing tolerances can usually be done rapidly if it has been previously checked or string lined by the Contractor. However, the time required to check subgrade material for design acceptance (PI and material passing a #200 [75?m] sieve) and to decide whether design changes are necessary could cause a delay in the acceptance of a section of subgrade. The Contractor should be kept informed of subgrade test status in order to modify the operations if necessary.

If at any time that hauling equipment leaves wheel impressions due to excessive moisture or drying, the subgrade needs to be refinished and re-compacted. It may also be advisable to retest for compaction.

All major work needed to complete the roadway to the design cross section should be completed before the subgrade is accepted. Major work includes operations that will cause re-excavation within an area where the base or pavement will be placed, or operations that will result in contamination or disturbance of the base or pavement.

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Examples of work that should be completed prior to aggregate base placement include:

? all cross drains and edge drains; ? all storm drain pipes; ? all conduit crossings for electrical, landscape irrigation, and freeway management; ? system components; and ? all utility relocation work.

Exhibit 300-1 shows a typical pavement section.

Finishing Tolerances

The standard specifications will provide the allowable tolerances that must be met by the Contractor in order for the Resident Engineer to accept the base or subbase work. The Inspector and the Contractor should string line the grade together for acceptance. Tolerances may vary depending upon the type of surface being placed (i.e., Portland cement concrete pavement or asphaltic concrete pavement).

TYPICAL PAVEMENT SECTION

A. Subgrade - The roadbed materials beneath the pavement structure. May be constructed from existing project materials or from imported borrow.

B. Finished Subgrade Elevation - The top prepared surface of the subgrade. C. Subbase Course - One or more layers of specified material of design thickness, placed on a subgrade to

support a base or surface course. A subbase is not often used. D. Base Course - One or more layers of specified material of designed thickness, placed on a subbase course

or a subgrade to support a surface course. E. Surface Course - The upper portion of the pavement structure consisting of a mixture of mineral aggregates

and bituminous material or Portland cement concrete. This includes all wearing courses (e.g. asphaltic concrete friction course). F. Pavement Structure - The combination of subbase course, base course, and surface course placed on the subgrade to support the traffic load.

F - PAVEMENT STRUCTURE

Exhibit 300-1. Typical Pavement Section

E - SURFACE CO URSE D - BASE CO URSE

C - SUBBASE CO URSE A - SUBG RADE

B - FINISHED SUBG RADE ELEVATIO N

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301 LIME TREATED SUBGRADE

The subgrade may be treated to improve its ability to support traffic loads. The improved load carry capability occurs because the lime admixture reduces the volume change characteristics and plasticity of the soil.

301-2 Materials

The Special Provisions will specify which type of lime to use. Two kinds of lime may be used for lime treatment: dry hydrated or commercial (granular or pelted) quicklime.

Dry hydrated lime has had water added and chemically combined. Quicklime has not been combined with water.

The Standard Specifications allow the use of lime from different sources, but not from different sources to be mixed. The problem with mixing lime from different sources is that they may react differently and result in nonuniformity. Hydrated lime and quicklime cannot be mixed together, even if they are from the same source, because of the very different water demands, curing time, and general handling procedures.

Whatever kind of lime is used, it must be kept dry and free flowing. Storage time should be as short as possible.

301-3 Construction Requirements

301-3.01 Preparation of Roadbed

Before adding lime, the subgrade material to be treated is scarified, pulverized, mixed, and windrowed or re-laid. It may be necessary to remove oversize material (see Standard Specification Subsection 301-3.01).

Windrows should be sized so that the mixer can easily process all the material at once. This may mean that several windrows will be needed. Running the mixer at its capacity limit, generally, means the mixer has to slow down to achieve thorough mixing. Even with the slow down, a nonuniform mix may result.

301-3.02 Application of Lime

The Special Provisions will specify the application rate for adding lime to the treated material. Whether applied to a windrow or on the shaped grade, the lime is applied by a mechanical device that can be adjusted to provide the correct spread within 10% of the amount specified.

Favorable weather is very important to the process. Cold weather will slow the reaction of the lime, and freezing will deprive the mix of the water needed. Rain can result in too much water that can either stop the process by dilution or wash out the lime. Windy conditions may cause safety and environmental problems especially when quick lime is used.

301-3.03 Mixing

Water is to be added only through the mixing machine so that optimum moisture can be closely controlled.

Quicklime needs more water than hydrated lime because the larger particles have to slake and become fine enough to hydrate. Slaking is the sloughing and falling apart of large pieces until only a fine powder results. A curing period is often necessary.

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The tolerance specification for the lime applied should be followed closely because the benefit of adding extra lime is not proportional to the amount added. A minor increase is unnecessarily expensive without a proportional increase in quality. A minor decrease in the lime greatly decreases the quality.

After mixing and laying, the finished subgrade is to be kept moist (not sloppy wet) until the bituminous curing seal is applied. The cure should be applied as soon as possible.

All traffic, including the Contractor's and ADOT's, must be kept off the treated area for three days.

301-3.04 Compaction and Finishing

Lime treated material should be compacted to a density of at least 100 percent of the maximum density unless the Special Provisions specify otherwise.

301-3.06 Safety Program

Both kinds of lime must be handled carefully because they can cause severe chemical burns, especially to the eyes. The Standard Specifications require the Contractor to have first-aid treatment available. The minimum precautions are described in the Standard Specifications.

The Resident Engineer is responsible for providing the same first-aid facilities for ADOT employees. If a satisfactory arrangement can be worked out, it is acceptable for ADOT personnel to use the Contractor's safety facilities.

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